Parts of a Circle

Every circle has several key parts that you'll need to know inside and out. The centre is the exact middle point of the circle - think of it as the bullseye on a dartboard.

The radius is any straight line from the centre to the edge of the circle. All radii (that's the plural of radius) are exactly the same length in any circle. The diameter goes right through the centre, connecting two points on the opposite sides - it's always twice as long as the radius.

The circumference is the distance all the way around the outside of the circle. An arc is just a portion of this circumference, like a slice of the outer edge. A chord is any straight line that connects two points on the circle's edge.

Finally, a tangent is a straight line that just touches the circle at exactly one point - it never crosses through the circle. Segments and sectors are the areas you get when you slice up a circle, with sectors being the "pizza slice" shapes that include the centre.

Quick Tip: Remember that the diameter is always exactly double the radius - this relationship will save you time in calculations!